Refuse a __main__ node where its body is written, not at import

A module defining nodes could not be run directly: the decorator refused
`__main__` while the module body was still executing, so a `if __name__ ==
"__main__"` self-check beside the nodes was impossible and the checks had
to live in a separate pytest file. The refusal now fires where the
generated body is written — document() and shims(), both, since sync writes
the document first — and the decorator hands the function back as it always
did. Syncing a __main__-defined node is still refused, with the same words.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/tests/sdk/test_build.py

@@ -6,7 +6,7 @@ from fluksio.flow.schemas import FlowDef
 from fluksio.sdk import MARKER, Flow, Port, SyncError, node, use
 
 # The functions live here rather than in each test: a node's module is what the
-# generated body imports, and `__main__` is refused for exactly that reason.
+# generated body imports, and `__main__` is refused at sync for that reason.
 
 
 @node(provides=[Port("dataset", "artifact"), Port("rows", "int")])
@@ -316,3 +316,27 @@ def test_a_negative_timeout_is_refused():
 def test_a_zero_timeout_means_no_limit():
     decorated = node(requires=["a"], timeout=0)(one_default)
     assert decorated.__fluksio__.timeout == 0
+
+
+def test_a_node_in_a_script_run_directly_is_refused_at_sync_not_at_import():
+    """A study module has to be runnable as a script for a self-check.
+
+    The refusal belongs where the body is generated: the decorator hands the
+    function back untouched, so `python study.py` declares its nodes, calls
+    them and checks itself. What cannot be done is syncing them, because
+    nothing could import `__main__`.
+    """
+
+    def probe(rows=1):
+        return {"score": float(rows)}
+
+    probe.__module__ = "__main__"
+    decorated = node(provides=[Port("score", "float")])(probe)
+    # Declared and callable, which is the whole point of running the file.
+    assert decorated(rows=2) == {"score": 2.0}
+
+    flow = Flow("mainflow", nodes=[decorated], outputs=["score"])
+    with pytest.raises(SyncError, match="run directly"):
+        flow.document()
+    with pytest.raises(SyncError, match="run directly"):
+        flow.shims()